A Morphometric Comparison of the Nuclear Morphology of Fresh and Frozen‐Thawed Human Zona‐Bound and Unbound Sperm

Abstract

The role of sperm nuclear morphology in zona binding and the effect of cryopreservation on sperm nuclear morphology have been investigated using objective criteria. The nuclear dimensions of fresh and frozen‐thawed sperm heads that had been prepared by the swim‐up (SU) technique, and bound to human oocytes in vitro, were measured using a computer‐assisted image analyzer. These were compared with each other and also to sperm in the SU samples and those that had not bound to the zona pellucida. The unbound (UB) sperm and SU sperm had very similar nuclear morphology. In contrast, the nuclear morphology of the zona‐bound (ZB) sperm differed significantly (P < 0.05) from that of the SU and UB sperm. These differences were found specifically between area, breadth, and roundness in the SU and ZB sperm preparations, and between area and roundness in the UB and ZB sperm preparations; the ZB sperm had smaller but rounder nuclei than either the SU or UB sperm. In addition, the frozen‐thawed sperm in the SU and UB groups showed a trend towards smaller nuclei than the corresponding fresh groups, whereas the frozen‐thawed ZB sperm had significantly (P < 0.05) smaller nuclear dimensions than the fresh ZB sperm. From the present study it appears that the ZB sperm are a morphologically distinct population, although it is not yet clear whether these differences reflect functional events that occur in the sperm head during its maturation or changes at the level of the sperm's DNA after the initial stage of zona binding. In addition, cryopreservation was found to cause a marked reduction in the head dimensions of the ZB sperm. Such a reduction might reflect a compaction of the sperm's DNA as a result of the freeze‐thawing process.